A semi-transmissive liquid crystal display device has a transmissive region for transmitting light from a backlight to provide transmission mode display and a reflective region for reflecting external light to provide reflection mode display in each pixel arranged in a matrix pattern as a minimum unit of an image. Therefore, the semi-transmissive liquid crystal display device can maintain a sufficient contrast and provide a high visual recognition property regardless of ambient brightness.
An active matrix-driven liquid crystal display device includes an active matrix substrate having a plurality of pixel electrodes arranged in a matrix pattern, a counter substrate provided so as to face the active matrix substrate and having a common electrode, and a liquid crystal layer provided between these substrates. In a semi-transmissive liquid crystal display device as described above, each pixel electrode includes a transparent electrode that forms the transmissive region and a reflective electrode that forms the reflective region. The reflective electrode is often made of a metal conductive film having a high reflectance such as an aluminum film, and the transparent electrode is often made of a transparent conductive film such as an ITO (Indium Tin Oxide) film and an IZO (Indium Zinc Oxide) film.
Each electrode material such as the metal conductive film and transparent conductive film has its own unique work function. Therefore, in the semi-transmissive liquid crystal display device, the reflective electrode and the transparent electrode have different work functions from each other. In this case, a surface potential is different between the reflective electrode and the transparent electrode, thereby causing flicker. This may result in significant degradation in display quality.
The reason why such flicker is generated will now be described.
In order to prevent image burn-in, a liquid crystal display device needs to be AC (alternating current)-driven by alternately applying a positive voltage and a negative voltage to a liquid crystal layer. More specifically, a positive voltage and a negative voltage are alternately applied to the liquid crystal layer by writing charges to pixel electrodes with the polarity of the charges being inverted at every prescribed period. At this time, an optimal counter potential is set to a common electrode of a counter substrate so that the positive and negative voltages that are applied to the liquid crystal layer become effectively equal to each other.
In a semi-transmissive liquid crystal display device, however, a surface potential becomes different between a reflective electrode and a transparent electrode due to the above-described difference in work function, and an optimal counter potential is set to only one of the reflective electrode and the transparent electrode. In this case, a direct-current voltage is applied to a liquid crystal layer in a region of the electrode to which the optimal counter potential is not set. Therefore, the positive and negative voltages applied to the liquid crystal layer become asymmetrical, whereby flicker, periodic luminance variation, is generated.
It is known in the art that generation of flicker is suppressed in a semi-transmissive liquid crystal display device when respective electrode materials in the reflective region and the transmissive region have the same work function.
For example, Patent document 1 discloses a liquid crystal display device in which a transparent electrode is provided in a reflective region and a transmissive region and a voltage is applied to a liquid crystal layer through the transparent electrode.
FIG. 39 is a schematic cross-sectional view of a semi-transmissive liquid crystal display device 150a corresponding to the semi-transmissive liquid crystal display device disclosed in FIG. 6 of Patent document 1.
As shown in FIG. 39, the semi-transmissive liquid crystal display device 150a includes an active matrix substrate 120a having pixel electrodes each formed by a reflective electrode 106a and a transparent electrode 107a, a counter substrate 130a provided so as to face the active matrix substrate 120a and having a common electrode 122, and a liquid crystal layer 125 provided between the substrates 120a and 130a. 
In the semi-transmissive liquid crystal display device 150a, an interlayer insulating film 112 for compensating for the optical path difference between a reflective region R and a transmissive region T is provided under the reflective electrode 106a. Therefore, the transparent electrode 107a is formed in the reflective region R and the transmissive region T through a stepped portion of the interlayer insulating film 112. Accordingly, when the transparent electrode 107a is thin, electric conduction may become defective in the stepped portion. When the transparent electrode 107a is thick, on the other hand, the reflectance of the reflective region R may be reduced.
A semi-transmissive liquid crystal display device 150b as shown in FIG. 40 is disclosed in FIG. 3 of Patent document 2.
In the semi-transmissive liquid crystal display device 150b, a first transparent electrode 102 made of an ITO film is formed on a substrate 110a. A reflective electrode 106a made of an aluminum film or the like and a second transparent electrode 107b made of an IZO film are sequentially formed on an interlayer insulating film 112 formed on the first transparent electrode 102. A region where the reflective electrode 106a and the second transparent electrode 107b are formed serves as a reflective region R, and a region exposed from the reflective electrode 106a in the first transparent electrode 102 serves as a transmissive region T.
The ITO film and the IZO film have a work function of 4.9 eV and 4.8 eV, respectively. The respective electrode materials in the reflective region R and the transmissive region T thus have almost the same work function. However, since flicker is slightly visually recognized, there is room for improvement. Note that, in view of transparency to visible light, conductive property, etching property and electrocorrosion with the underlying aluminum film, and the like, the IZO film is preferably used as the second transparent electrode 107b that covers the reflective electrode 106b. Therefore, the second transparent electrode 107b is not formed in the transmissive region T if priority is given to the efficiency of the manufacturing process.
Patent document 1: Japanese Laid-Open Patent Publication No. 2003-255378
Patent document 2: Japanese Laid-Open Patent Publication No. 2004-191958